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Posted by on Tuesday, March 27, 2012 at 6:01pm.

Calculate the change in PH when 6 ml of 0.1 M HCl is added to 100 ml of a buffer solution that is 0.1 M in NH3 and 0.1 M in NH4Cl. And calculate the change in PH when 6 ml of 0.1 M NaOH is added to the original buffer solution.

  • Chemistry - , Tuesday, March 27, 2012 at 7:24pm

    100 mL x 0.1M NH3 = 10 millimols NH3.
    100 mL x 0.1M NH4Cl = 10 mmols NH4Cl.
    6 mL x 0.1M HCl = 0.6 mmols HCl.
    6 mL x 0.1M NaOH = 0.6 mmols NaOH.

    ...........NH3 + HCl --> NH4^+ + Cl^-
    initial...10......0......0.1
    added...........0.6............
    change..-0.6...-0.6.......+0.6
    equil....9.4......0......0.7....

    Use the equilibrium values from the ICE chart, substitute them into the Henderson-Hasselbalch equation and solve for pH.

    For the addition of NaOH, it's the same concept BUT you want this equilibrium:
    ...........NH4^+ + OH^- ==>NH3
    Post your work if you get stuck.

  • Chemistry - , Tuesday, March 27, 2012 at 8:21pm

    I did it in moles but still the homework assignment website is not accepting these answers.
    My work:
    Initial PH = 9.24.
    moles of NH3= 0.01
    moles of NH4= 0.01
    After addition of HCl:
    moles of HCl added= 0.0006
    moles of NH3 left= 0.0094
    moles of NH4= 0.0106
    PH= 9.19

    Addition of NaOH:
    moles of Naoh= 0.0006
    moles of NH4= 0.0094
    moles of NH3= 0.0106
    PH= 9.30

    used HH equation in all, considering PKa of NH3= 9.25

  • Chemistry - , Wednesday, March 28, 2012 at 12:00am

    I suspect the problem is that the question asks for CHANGE in pH and you're giving the data base the pH. I think something like 1.128 which I would round to 1.13 is the answer for the HCl part. I didn't work the NaOH part.
    pH = pKa + log(9.24/0.7).
    pH = pKa + 1.128. I didn't calculate the final pH since the change is what you want.

  • Chemistry - , Wednesday, March 28, 2012 at 12:52am

    Still wrong answer.

  • Chemistry - , Wednesday, March 28, 2012 at 1:04am

    You're right. That isn't th right answer. You should have checked my work. The error I made is I wrote 0.1 mol for NH4^+ and it isn't that to start.
    It's 10............10
    ....-0.6............+0.6
    final.9.4...........10.6
    log(9.4/10.6) = ? and that's your answer.

  • Chemistry - , Wednesday, March 28, 2012 at 12:06pm

    I got initial PH= 9.24
    PH after HCl addition= 9.19
    PH after NaOH addition= 9.3
    so delta PH after HCl= 0.05
    delta PH after NaOH= - 0.06
    all these still wrong answers.
    could it be that I'm calculating the total volume wrong? should be 0.01+0.006 L?

  • Chemistry - , Thursday, March 29, 2012 at 1:03am

    I just took a quick look and I'm glad I did. I think you are working it right. You see my last response gives a value of 0.05218 (it should be -0.05218); you may not have keyed in enough significant figures. Round -0.05218 to the right number of s.f. from your original numbers.
    The other suggestion I have is that one of the answers may be right but the other wrong. Are you keying in both answers at the same time. That will tell you if that could be the problem. No, I don't think you can calculate the wrong volume; even if you put in a wrong volume it won't make any difference if you use the same volume for both acid and base. That's why I work in millimols because mmols/mL = M but since mL is the same for both acid and base the volume always cancels.
    I worked the base addition and the answer is the same except for a + number.
    ...........NH4^+ + OH^- ==> NH3
    initial.....10...............10
    change......-0.6..............+0.6
    equil......9.4...............10.6
    log b/a = log (10.6/9.4) = 0.05218
    I have omitted the subtraction step since the initial pH is
    pH = pKa + log(b/a) = pKa + 0 so the difference between pH of one soln and the original is just the log b/a term and I need not go through the subtraction step. But note this and this could be the problem. One of those 6 mL additions causes a +0.05218 pH difference and the other 6mL addition causes a -0.05218 pH difference. Are you keying in the negative sign? If you are allowed only 1 s.f. (from the 6 mL but I don't know if you just didn't type in all of the zeros) then +0.05 and -0.05 would be the answers. Parenthetically, I obtained 9.197 for the 9.19 you have and that rounds to 9.20 and would account for you obtaining 0.05 for one answer and 0.06 for the other. I'm going with 0.05 for one and -0.05 for the other if the 6 mL addition allows just 1 s.f. for the answer.
    Please post a NEW post to me at the top of the page when you get it done. This page is getting so far back it's hard to find.

  • Chemistry - , Thursday, March 29, 2012 at 10:44am

    You are right, thank you so so much.
    With addition of HCl, the Ph= -0.052.
    With addition of NAOH, the PH= 0.052.
    Thank you for all your efforts.

  • Chemistry - , Thursday, March 29, 2012 at 12:26pm

    With pleasure.

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